Love this lens. I picked it up when the Mark II was in production. Having saved thousands, I finally felt like it was worth it from the cost to benefit standpoint. I don't do a lot of sports, but 10 years from now when I have kids running around on the field, this lens will be used non-stop.

For now, depending on the situation, I use it for extreme portraiture and even break it out at weddings every once and a while. I set it up to capture the first kiss from a distance, and fire it remotely with my radiopoppers. It allows me to get that tight shot without the minister/preacher in the background, while I grab the wide angle from really close.

Overall, this lens is a gem. If you can find the Mark I with IS for around the $5k price point, I would pick it up. The build quality is sure to last and the picture difference between it and the II is hardly noticeable. It is heavy, but thats just understood when shooting 400/2.8

Long time 5D Mark I and II user. Loved the sensor in each revision, but hated the slow/clunky feeling to the camera. The 5D III takes care of this. Canon finally put a professional touch on it. Granted, it doesn't compete speed wise with the 1DX, but I hardly ever need a camera that fast.

For my line of work, wedding photographer, the 5D III is absolutely the best camera for the job. Lots of colleagues using the D800, which is a solid machine too, but having bought into the Canon ecosystem a long time ago, I couldn't be more satisfied with my equipment setup.